Cancer is the leading cause of death worldwide. In 2005, cancer accounted for 7.6 million (or 13% of all) deaths. Cancer stem cells have been prospectively identified in several human cancers. From a functional standpoint, these cells are highly specialized since they have the unique capacity to undergo long-term proliferation. Therefore, cancer stem cells are thought to represent the cellular component within human cancers that are ultimately responsible for growth during both initial tumor formation and disease relapse.